An exchange used in the conference system of the prior art includes register means for registering the calling numbers of plural telephones as one group from the outside so that all the telephones can communicate by calling all the telephones belonging to the group in response to the call of a special number from the telephone of one calling number belonging to that group (as referred to JP-A-2000-36873).
This conference system exchange, as disclosed in Patent Publication 1, is provided with registration means capable of registering the calling numbers of the telephones belonging to the group from the outside so that the participants of the conference can be freely set.